As portable devices have come into widespread use, the demands for small secondary batteries having a large capacity have increased in recent years. Among secondary batteries currently in practical use, lithium on secondary batteries have been known as batteries having the highest energy density. In such a conventional lithium ion secondary battery, an organic solvent electrolyte which exhibits a high ion conductivity is used as an electrolyte. However, the organic solvent electrolyte is liquid and flammable. Hence, when used as an electrolyte for a lithium ion secondary battery, the organic solvent electrolyte poses risks for developing leakage of the liquid, ignition or the like. There is a safety concern about the use of the organic solvent electrolyte. For this reason, a safer solid electrolyte has been demanded as an electrolyte for a lithium ion secondary battery.
Polymer-based, oxide-based, and sulfide-based solid electrolytes and the like have been known as solid electrolytes having a lithium ion conductivity. For example, Japanese Unexamined Patent Application Publication No. 2005-228570 (PLT 1) discloses a solid electrolyte obtained by using, as a raw material, a sulfide-based crystallized glass containing elements of lithium, phosphorus, and sulfur, which has a lithium ion conductivity. Meanwhile, Japanese Unexamined Patent Application Publication No. 2007-273217 (PLT 2) discloses a solid electrolyte containing elements of lithium, phosphorus, sulfur, and oxygen, as well as another element belonging to any one of Groups 13 to 16 of the periodic table (provided that phosphorus, sulfur, and oxygen are, excluded).
Meanwhile, the present inventors have found that LiBH4 exhibits a high ion conductivity at high temperatures of approximately 117° C. (approximately 390 K) and above (M. Matsuo, Y. Nakamori, S. Orimo, H. Maekawa, and H. Takamura, Appl. Phys. Lett., 2007, vol. 91, 224103 (NPL 1)). However, LiBH4 has a large resistance below 115° C. (308 K), which is a transition temperature thereof from a high-ion-conductive phase (a high-temperature phase). Accordingly, the use of LiBH4 as an electrolyte for a lithium ion secondary battery involves a problem of a low ion conductivity at low temperatures, in particular, at around room temperature.